# Customizing Gappa¶

These sections explain how rounding operators and back-ends are defined
in the tool. They are meant for developers rather than users of Gappa and
involve manipulating C++ classes defined in the `src/arithmetic`

and `src/backends`

directories.

## Defining a generator for a new formal system¶

To be written.

## Defining rounding operators for a new arithmetic¶

### Function classes¶

A function derives from the `function_class`

class. This class is an
interface to the name of the function, its associated real operator, and
six theorems.

```
struct function_class
{
function_class(real_op_type t, int mask);
virtual interval round (interval const &, std::string &) const;
virtual interval enforce (interval const &, std::string &) const;
virtual interval absolute_error (std::string &) const;
virtual interval relative_error (std::string &) const;
virtual interval absolute_error_from_exact_bnd (interval const &, std::string &) const;
virtual interval absolute_error_from_exact_abs (interval const &, std::string &) const;
virtual interval absolute_error_from_approx_bnd(interval const &, std::string &) const;
virtual interval absolute_error_from_approx_abs(interval const &, std::string &) const;
virtual interval relative_error_from_exact_bnd (interval const &, std::string &) const;
virtual interval relative_error_from_exact_abs (interval const &, std::string &) const;
virtual interval relative_error_from_approx_bnd(interval const &, std::string &) const;
virtual interval relative_error_from_approx_abs(interval const &, std::string &) const;
virtual std::string description() const = 0;
virtual std::string pretty_name() const = 0;
virtual ~function_class();
};
```

The `description`

function should return the internal name of the
rounding operator. It will be used when generating the notations of the
proof. When the generated notation cannot be reduced to a simple name,
comma-separated additional parameters can be appended. The back-end will
take care of formatting the final string. This remark also applies to
names returned by the theorem methods (see below). The `pretty_name`

function returns a name that can be used in messages displayed to the
user. Ideally, this string can be reused in an input script.

The `real_op_type`

value is the associated real operator. This will be
`UOP_ID`

(the unary identity function) for standard rounding
operators. But it can be more complex if needed:

```
enum real_op_type { UOP_ID, UOP_NEG, UOP_ABS, BOP_ADD, BOP_SUB, BOP_MUL, BOP_DIV, ... };
```

The type will indicate to the parser the number of arguments the
function requires. For example, if the `BOP_DIAM`

type is associated
to the function `f`

, then `f`

will be parsed as a binary function.
But the type is also used by the rewriting engines in order to derive
default rules for this function. These rules involve the associated real
operator (the diamond in this example).

For these rules and the following theorems to be useful, the expressions \(f(a, b)\) and \(a \diamond b\) have to be close to each other. Bounding their distance is the purpose of the last ten theorems. The first two theorems compute the range of \(f(a, b)\) itself.

It is better for the proof engine not to consider theorems that never
return a useful range. The `mask`

argument of the `function_class`

constructor is a combination of the following flags. They indicate which
theorems are known. The corresponding methods should therefore have been
overloaded.

```
struct function_class
{
static const int TH_RND, TH_ENF, TH_ABS, TH_REL,
TH_ABS_EXA_BND, TH_ABS_EXA_ABS, TH_ABS_APX_BND, TH_ABS_APX_ABS,
TH_REL_EXA_BND, TH_REL_EXA_ABS, TH_REL_APX_BND, TH_REL_APX_ABS;
};
```

All the virtual methods for theorems have a similar specification. If
the result is the undefined interval `interval()`

, the theorem does
not apply. Otherwise, the last parameter is updated with the name of the
theorem that was used for computing the returned interval. The proof
generator will then generate an internal node from the two intervals and
the name. When defining a new rounding operator, overloading does not
have to be comprehensive; some functions may be ignored and the engine
will work around the missing theorems.

`round`

- Given the range of \(a \diamond b\), compute the range of \(f(a, b)\).
`enforce`

- Given the range of \(f(a, b)\), compute a stricter range of it.
`absolute_error`

- Given no range, compute the range of \(f(a, b) - a \diamond b\).
`relative_error`

- Given no range, compute the range of \(\frac{f(a, b) - a \diamond b}{a \diamond b}\).
`absolute_error_from_exact_bnd`

- Given the range of \(a \diamond b\), compute the range of \(f(a, b) - a \diamond b\).
`absolute_error_from_exact_abs`

- Given the range of \(|a \diamond b|\), compute the range of \(f(a, b) - a \diamond b\).
`absolute_error_from_approx_bnd`

- Given the range of \(f(a, b)\), compute the range of \(f(a, b) - a \diamond b\).
`absolute_error_from_approx_abs`

- Given the range of \(|f(a, b)|\), compute the range of \(f(a, b) - a \diamond b\).
`relative_error_from_exact_bnd`

- Given the range of \(a \diamond b\), compute the range of \(\frac{f(a, b) - a \diamond b}{a \diamond b}\).
`relative_error_from_exact_abs`

- Given the range of \(|a \diamond b|\), compute the range of \(\frac{f(a, b) - a \diamond b}{a \diamond b}\).
`relative_error_from_approx_bnd`

- Given the range of \(f(a, b)\), compute the range of \(\frac{f(a, b) - a \diamond b}{a \diamond b}\).
`relative_error_from_approx_abs`

- Given the range of \(|f(a, b)|\), compute the range of \(\frac{f(a, b) - a \diamond b}{a \diamond b}\).

The `enforce`

theorem is meant to trim the bounds of a range. For
example, if this expression is an integer between 1.7 and 3.5, then it
is also a real number between 2 and 3. This property is especially
useful when doing a dichotomy resolution, since some of the smaller
intervals may be reduced to a single exact value through this theorem.

Since the undefined interval is used when a theorem does not apply, it
cannot be used by `enforce`

to flag an empty interval in case of
a contradiction. The method should instead return an interval that does
not intersect the initial interval. Due to formal certification
considerations, it should however be in the rounded outward version of
the initial interval. For example, if the expression is an integer
between 1.3 and 1.7, then the method should return an interval contained
in \([1,1.3)\) or \((1.7,2]\). For practical
reasons, \([1,1]\) and \([2,2]\) are the most interesting
answers.

### Function generators¶

Because functions can be templated by parameters. They have to be
generated by the parser on the fly. This is done by invoking the
functional method of an object derived from the `function_generator`

class. For identical parameters, the same `function_class`

object
should be returned, which means that they have to be cached.

```
struct function_generator {
function_generator(const char *);
virtual function_class const *operator()(function_params const &) const = 0;
virtual ~function_generator() {}
};
```

The constructor of this class requires the name of the function
template, so that it gets registered by the parser. `operator()`

is
called with a vector of encoded parameters.

If a function has no template parameters, the
`default_function_generator`

class can be used instead to register it.
The first parameter of the constructor is the function name. The second
one is the address of the `function_class`

object.

```
default_function_generator::default_function_generator(const char *, function_class const *);
```